Recently in the electronics industry there has been a demand for an increase in integration thus requiring a photo resist having a higher resolving power. In addition, a higher sensitivity is demanded so as to shorten the time required for production steps.
Studies for enhancing sensitivity of a photo-polymerizable composition have conventionally been conducted and, as a result, many photo-polymerization initiators (systems) have been proposed. For example, U.S. Pat. No. 2,448,828 describes benzoin ethers, U.S. Pat. No. 3,046,127 describes anthraquinones, JP-B-49-11936 (the term "JP-B" as used herein means an "examined Japanese patent publication") (corresponding to U.S. Pat. No. 3,661,588) describes aminophenylketone and an active methylene or amino compound, U.S. Pat. No. 3,682,641 describes Michler's ketones and benzophenone, JP-B-48-38403 describes benzophenone and 4-N,N-diethylaminobenzophenone, JP-B-45-37377 and JP-B-48-38403 describe Michler's ketones and lophine dimers, JP-A-56-35134 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes lophine dimers and photo-polymerizable unsaturated monomers, JP-A-59-78339 describes a ternary system of 4-N,N-dialkylaminobenzophenones, benzophenone and an organic halogen compound, and the like.
It is true that the use of these photo-polymerization initiator systems serves to improve sensitivity and resolving power to some extent, but the degree of improvement is still insufficient for satisfying the recent requirements. In addition, when used as dry film resists to be used upon preparation of printed circuit boards, they provide a tenting film with only an insufficient strength upon formation of through-holes according to the tenting process, and hence a film-breaking trouble has been liable to take place during a development or etching step. It is well known that breakage of a film during the development or etching step can cause a serious problem of conduction failure as a result of dissolution with an etching solution of the copper-plated portion inside the through-hole conducting the surface side and the back side to each other.